The coating of electroconductive substrates by electrodeposition is an important industrial process. In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous dispersion of film-forming polymer. An electric current is passed between the article and a counter electrode in electrical contact with the aqueous dispersion until a desired amount of coating is produced on the article. The article to be coated can made the anode or the cathode depending upon the ionic nature of the coating system.
Cationic coating compositions generally are derived from resinous compositions containing a basic nitrogen atom which can be neutralized with an acid and then be dissolved or dispersed in water. Sufficient basic nitrogen atoms should be present so that the dispersibility or solubility can be obtained with a minimum amount of acid. The most commonly employed type of cathodically electrode depositable resins are made by reacting a polyepoxide resin with amine at elevated temperatures, as exemplified in U.S. Pat. No. 4,137,140 to Belanger, U.S. Pat. No. 4,182,831 to Hicks, and U.S. Pat. Nos. 3,922,253 and 3,947,338 to Jerabek et al. Marchetti et al in U.S. Pat. No. 4,104,147 disclose chain-extended polyepoxide which is adducted with a secondary amine, wherein the chain-extended polyepoxide is made by reacting a high molecular weight epoxide with an organic polyol. May, in U.S. Pat. No. 3,719,626 discloses amine adducts of polyepoxides, wherein the polyepoxide includes epoxidized polyesters obtained by reacting an unsaturated polyhydric alcohol and/or an unsaturated polycarboxylic acid. Aqueous coating compositions of these resins are made by reacting the resin with a water soluble acid and subsequently dispersing the resin in water. The coating compositions are deposited on a substrate and crosslinked at elevated temperatures by means of an agent which is capable of reacting with the hydroxyl or amine functionality of the resin during curing to form a thermoset film. The crosslinking agent, which is generally a blocked isocyanate, may be incorporated into the resin molecule as is taught in the Jerabek et al patents or, as is more generally the case, the crosslinking agent may be present in the aqueous coating bath so as to codeposit with the resin.